LED Resistor Calculator
An LED Resistor Calculator is a tool that determines the correct resistor value needed to safely operate an LED with a given power source. LEDs (Light Emitting Diodes) are sensitive semiconductor devices that require precise current control to function properly and avoid damage.
LEDs have very low internal resistance. When connected directly to a power source like a battery or USB port, they act almost like a short circuit and draw excessive current. This instantly overheats and destroys the LED. A resistor limits the current to a safe level (typically 20mA) while dropping the excess voltage.
Key concepts this calculator handles:
- Voltage Difference: Calculates voltage drop across resistor (Vs - Vf)
- Current Limiting: Sets safe operating current (usually 10-30mA)
- Power Rating: Determines resistor wattage needed
- Standard Values: Finds nearest available resistor
Follow these three simple steps to calculate the perfect resistor for your LED:
- Supply Voltage (Vs): Your power source voltage (e.g., 5V USB, 9V battery, 12V car battery)
- LED Voltage (Vf): Forward voltage of your LED (depends on color - see table below)
- LED Current (I): Desired operating current in milliamps (mA) - typically 20mA
The calculator instantly provides:
- Exact resistor value needed (in Ohms)
- Nearest standard resistor value available
- Power dissipation (wattage) for resistor selection
- Recommended resistor wattage (¼W, ½W, etc.)
Different LED colors have different forward voltage requirements. Use this table to find your LED's voltage (Vf):
| LED Color | Forward Voltage (Vf) | Typical Current | Common Applications |
|---|---|---|---|
| Red | 1.8 - 2.2V | 15-20mA | Power indicators, status lights |
| Green | 2.0 - 3.0V | 15-20mA | Power lights, go indicators |
| Blue | 3.0 - 3.6V | 20-30mA | Decoration, accent lighting |
| White | 3.0 - 3.6V | 20-30mA | General lighting, flashlights |
| Yellow/Amber | 1.8 - 2.2V | 15-20mA | Warning lights, caution indicators |
If your LED voltage is not listed or you have the datasheet, select "Custom" in the calculator and enter the exact forward voltage from your LED specifications.
Below are answers to frequently asked questions about LED resistor selection and circuit design:
This happens when no resistor or wrong value is used. Calculate with:
- Supply voltage (Vs): 5V
- LED voltage (Vf): 2V for red LED
- Current (I): 20mA = 0.02A
- Formula: (5 - 2) ÷ 0.02 = 150Ω
- Use 150Ω resistor in series with LED
Enter your values in calculator above for exact resistor needed.
Car batteries are 12V-14V, too high for direct LED connection.
- Vs: 12V
- Vf: 3.3V for white LED
- I: 20mA = 0.02A
- Calculation: (12 - 3.3) ÷ 0.02 = 435Ω
- Use 470Ω resistor (nearest standard)
- Resistor power: ¼ Watt minimum
Arduino Uno outputs 5V from its pins. Common values:
| LED Color | Voltage (Vf) | Resistor Needed |
|---|---|---|
| Red | 2.0V | 150Ω |
| Green | 2.2V | 140Ω |
| Blue | 3.2V | 90Ω |
| White | 3.3V | 85Ω |
All calculations based on 20mA current. Use calculator above for exact values.
Yes, but Arduino pins have 40mA limit. Options:
- Use 10mA per LED instead of 20mA
- Total: 10mA × 3 = 30mA (safe)
- Recalculate with I=10mA in calculator
- For red LEDs: (5-2)÷0.01 = 300Ω each
- Or use transistor to drive more LEDs
First calculate resistor, then battery life:
- Resistor: (3-2)÷0.02 = 50Ω
- AA battery capacity: ~2000mAh
- Current draw: 20mA
- Battery life: 2000 ÷ 20 = 100 hours
- For longer life, use 10mA current
This is normal with resistor circuits. As battery voltage drops:
- Less voltage across resistor = less current
- LED brightness depends on current
- Example: 9V battery drops to 7V → LED gets dimmer
Solutions: Use fresh batteries, add more batteries in parallel, or use constant-current LED driver for critical projects.